But consider what Asimov had to work with here. He had four stories he'd written at various times in his career being published in a single book—not to mention a brand new fifth story. Each story contains different characters living in different eras not to mention different plots and different themes. About the only similarity these tales maintain is the setting of the Foundation… Bingo!
So, Asimov titled the novel based on the key feature shared by all the stories: the Foundation. And it's a naming convention he stuck with in the sequels. In the second novel of the trilogy, Foundation and Empire, all the stories focus on the conflicts between the Foundation and—you guessed it—the remnants of the Galactic Empire. And final novel in the trilogy, Second Foundation, finally brings to light the mysteries surrounding the second Foundation.
Straightforward? Maybe. But that doesn't mean it's simple.